Study On The Physical And Mechanical Properties Of Wall Rock During The Exploration Of Hot Dry Rock

With the continuously increased population growth and fast developing economy,the fossil fuels such as coal,oil and natural gas,with the resulting release of acid rain and greenhouse gases,become increasingly depleted.Clean and efficient energy is the direction of future energy development,among which deep geothermal energy offers great advantages in cost,reliability and environmental friendliness.Thus,deep geothermal energy exploitation is now recommended and identified as a renewable and alternative energy source.Aiming to extract the energy stored in deep geothermal energy,Enhanced Geothermal System(EGS)that recovers heat from an adequately hot thermal reservoir at shallow depth by creating an artificial circulation system was designed and implemented by Los Alamos National Laboratory in the 1970s.EGS entails using hydraulic fracturing and simulation to create highly conductive zones that interconnect an injection well to a production well,forming a doublet system.Thermal energy is extracted by circulating water,or another appropriate fluid,into the hot fractured rock and then pumping it to a power plant on the surface to generate electricity.During the exploration of geothermal energy,cold water may be injected and circulated through the fractures in the geothermal reservoirs,which may change the properties of geothermal reservoir rocks(rocks at elevated temperatures)and further influence the stability and safety of geothermal wellbore wall.Thus it is of great significance to study the physical and mechanical properties of cyclic heat-treated granite by water cooling.In addition,rock masses are often under an unloading confining stress state during and after drilling,which can also induce severe rock failures.Loading and unloading stress conditions may induce reactivation of existing faults,generation of fracture networks and even seismic events.Therefore,it is essential to understand the mechanism of the combined effect of high temperature and loading/unloading stress conditions on deep rocks for successful implementation of subsurface utilization projects such as those above.Therefore,in this paper,thermal,drillability and loading/unloading mechanical tests were carried on the NA granite after high temperature,and mechanical tests were conducted on the SZ granite after water and cyclic cooling treatments.The change mechanisms of granites after thermal treatments were revealed by the SEM and optical microscopy observations on the microstructures of granites.Based on the mechanical tests,a statistical thermal damage constitutive model is developed to describe the stress-strain relations of granite after high temperature under loading and unloading conditions,and the rationality of this model was verified by comparing the calculated results with the experimental results.This research is hoped to provide a theoretical basis for geothermal simulation and engineering design of deep HDR drilling stability,and aid for wellbore stability and development and utilization of dry hot rock.The main content of this paper is divided into six chapters as following:The first chapter,the meaning of geothermal resources was firstly introduced,and the energy and environmental significance of geothermal resources,especially hot dry rock,was emphasized.The research purpose and significance were then illustrated.After that,the domestic and foreign research status of physico-mechanical properties of wellbore wall during drilling and exploitation of hot dry rock were overviewed,and the difficulties and problems faced with during drilling were analyzed.Finally,the key scientific problems,main content and technology route were introduced.The second chapter,the volume,mass and density of NA granite under and after high temperature were measured,and the P-wave of NA granite after air and water cooling was obtained.The change mechanism of physical properties of NA granite with thermal temperature was revealed by optical microscopy observation.The volume,mass,density and P-wave of SZ granite after cyclic water cooling treatments were measured,and the change mechanism of physical properties of SZ granite with thermal temperature and cyclic times was revealed according to SEM observation.The third chapter,the thermal expansion,thermal conductivity,thermal diffusivity and specific heat capacity of NA granite after high temperature were measured,and the changes in thermal properties of granite after air and water cooling treatments were analyzed.On the basis of optical microcopy observation,the change mechanism of thermal properties after thermal treatment was discovered.The fourth chapter,the uniaxial compression and micro-drilling tests were conducted on the NA granite after air and water cooling treatments,and the effects of thermal treatment and water cooling on NA granite were summarized.The conventional triaxial loading and unloading tests were carried on NA granite after high temperature,the effects of temperature,confining stress and loading path on the mechanical properties of NA granite were analyzed and their influence mechanism were revealed based on the optical microcopy observation.The uniaxial compression tests were conducted on SZ granite after cyclic water cooling treatments,and the change mechanism of mechanical properties of SZ granite with thermal temperature and cyclic times was revealed according to SEM observation.The fifth chapter,on the basis of Lemaitre's strain equivalence theory,the effects of high temperatures on the mechanical parameters of rock are taken into account,and a thermal damage variable D_T is introduced.A statistical thermal damage constitutive model is developed with the hypothesis that the mesoscopic unit strength of rock subjected to thermal treatment obeys a Normal random distribution.The values of the model parameters are determined by the extreme value method according to the concept of yielding.Furthermore,combined with the effective medium theory,the statistical damage constitutive model of rock after high temperature is improved considering the stage of pore compaction under the condition of loading and unloading.The rationality of the improved model is verified by comparing the calculated results with experimental results of NA granite after high temperature.The sixth chapter,the main conclusions and innovations of this paper were given,and the shortcomings and future research direction were summarized.Through the above research,the following conclusions are obtained:(1)The bulk volume and specific heat capacity of NA granite after high temperature increase with temperature and increase by 1.60%and 15.46%at 600?,while the P-wave,mass,density,thermal conductivity and thermal diffusivity decrease with temperature and decrease by 73.9%,0.23%,1.80%,30.71%and 34.93%at 600?.At the same temperature,the changes in physical properties of NA granite under high temperature are larger than those after high temperature and the variations in mass,bulk volume and density of NA granite under two conductions at 600?are0.09%,2.72%and 2.65%respectively.(2)The mechanical properties of NA granite after high temperature decrease with increasing temperature,and granite specimens are less brittle and have higher ductility and plasticity with temperature.At 600?,the conventional triaxial compressive strength under different confning stresses of 20,40 and 60 MPa decrease by 55.4%,58.6%and 49.5%,conventional triaxial elastic mudulous decrease by 45.9%,39.7%and 33.5%.Compared with the results of conventional triaxial tests,the mechanical properties of NA granite under unloading condition are degraded,and the capacity of NA granite is reduced.At 600?,the unloading triaxial compressive strength under different confning stresses of 20,40 and 60 MPa decrease by 53.9%,59.1%and49.8%,unloading triaxial elastic mudulous decrease by 40.2%,44.9%and 25.4%.(3)According the optical microcopy observation of NA granite after high temperature,the density and average width of microcracks increase with temperature.At 600?,the density and average width of microcracks are 1.97 mm/mm~2 and 25.16?m and a microcrack network is formed in the thin section.The deterioration of physico-mechanical properties is mainly due to the evolution of thermal microcracks and the change of microstructure of various minerals.The unloading stress state is equivalent to superimposing a lateral tensile stress on the loading stress state.This lateral tensile stress induces the extension of tension cracks parallel to the axial direction and degrades the mechanical characteristics of the granite.400?can be treated as the threshold temperature for significant deterioration,and temperatures above 400?have a greater effect on the strength and deformation properties of granite than unloading treatment.(4)The drilling rate,plasticity coefficient,and the mass losses of granite specimens after heating and water cooling and the grinder increase exponentially,and increase by 89.71%,81.65%,185.58%and 57.95%at 500°C,respectively.The curves of load-indentation depth for granite specimens transform from brittle to ductile with thermal temperature.At 600°C,the wellbore wall of granite collapsed during the micro-drilling test and friction and wear test.The drillability of granite samples under different heating and water-cooling conditions increases with thermal temperature.The improvement of drillability of granite is beneficial to drilling of EGS,while it will also cause wellbore instability during deep well drilling.(5)The volume of SZ granite after cyclic water cooling increases with thermal cycle,while mass density,P wave velocity,uniaxial compressive strength and elastic modulus decrease with thermal cycle,especially after the first thermal cycling.At500?,uniaxial compressive strength and elastic modulus decrease by 33.37%and38.82%after the first thermal cycling,respectively.Their change rates diminish with increasing thermal cycle and they remain almost unchanged once the thermal cycles reach 5 iterations.At 500?,uniaxial compressive strength decrease by 68.47%,77.64%and 38.82%,and elastic modulus decrease by 76.93%,88.17%and 88.42%after 10,20 and 30 iterations,respectively.Through the analysis of optical microscopy images after high temperature,it is found that the density and width of microcracks increase with thermal cycle,which is consistent with the change trend of physico-mechanical properties of granite with thermal cycle.At 500?,the densities of microcracks of SZ granite after 1,5,10,20 and 30 iterations are 4.90,5.34,6.07,7.33 and 9.03 times as much as that under normal temperature,respectively.The deterioration of the physical and mechanical characteristics of granite after thermal cycling treatment with water cooling is mainly dependent on the generation and propagation of microcracks.(6)Under uniaxial compression test,the failure mode of NA granite after high temperature changes from axial splitting to shear failure.At 600?,the failure mode of NA granite is shear failure.Under conventional triaxial loading,NA granite after high temperature presents shear failure.Confining pressure has little effect on the failure mode of rock,but it makes the damaged rock more broken.Due to unloading path,NA granite after different high temperatures mainly shows double shear failure,since the unloading stress state is equivalent to superimposing a lateral tensile stress on the loading stress state.Without thermal treatment,SZ granite presents an axial splitting failure mode.With the increase of temperature and cycle times,the failure mode of SZ granite transforms from axial splitting to shear failure,and the integrity of the granite specimens decreases gradually.(7)The deformation of rocks after high temperature is divided into void and skeleton portions,and they were considered in the establishment of the proposed statistical thermal damage constitutive model.The theoretical results of the damage deformation model considering characteristics of the void compaction stage show good agreement with experimental data on conventional compression tests on Nanan granite after exposure to various temperatures.The model parameters involved in the proposed model can be obtained from conventional rock mechanical tests,which is convenient in engineering applications.The normal statistical distribution and mechanical parameters are lithology-independent and universal for different rocks.